Storm water runoff can carry sediment from soil erosion and other residues from a retention pond or other water holding area if it is allowed to be released too rapidly.
The adverse effects of such uncontrolled storm runoff effluents are well documented. The Federal Clean Water Act (CWA) regulates storm water discharge through the National Pollutant Discharge Elimination System (NPDES) that require a storm water pollution prevention plan (SWP3) to be prepared for each site. The post-construction best management practices (BMP's) require a 48 hour draw down time for extended detention basins. (dry basins). The longer draw down period for storm water discharges are for water quality purposes.
This regulation mandates a rate of draw down that is substantially slower than previously allowed. The benefits of such a draw down rate are believed to be providing more time for small particle contaminants to settle in the detention basin bed.
The primary problem is there has been no catch basin overflow structures built or designed to handle such slow rates of discharge. Accordingly, a simple and efficient way to convert or retrofit existing structures to meet the new standards is needed. Similarly new structures need to be developed that can be designed with controlled runoff rates based on the surrounding requirements of the detention basin or ponds and therefore each new system ideally would be able to be custom sized for the conditions to achieve the desired rate of storm water draw down.
Several approaches to achieving controlled rates of flow have been attempted in storm water drainage systems. U.S. Pat. No. 4,522,533 discloses a tapered flow restriction with a cover plate having a predetermined aperture at an end. The tapered part being inserted into the end of a sewer pipe. The flow restrictor is used to prevent storm water backup in urban sewer systems which results in flooding of basements and other significant inconveniences. Similarly U.S. Pat. No. 5,080,137 teaches Vortex Flow Regulators for Storm Sewer Catch Basins, the flow being controlled by a spiraled shape to restrict the rate of flow initially and which increases in area outwardly along the spiral permitting large volume flows to the catch basin or manhole to be accommodated where it is installed. The spiral flow was believed to be less prone to clogging. U.S. Pat. No. 3,938,713 taught a Flow Regulator for sediment collecting chambers of a separating device.
None of these devices provides a way to optimally size or control the draw down rate for an overall catch basin system at rates of outflow less than 1.0 cfs.
One system used a plurality of conventional rip rap filled with gabion boxes aligned end to end to restrict the rate of flow of storm water runoff in areas under construction wherein high mud levels were commonly found. The problem with this flow restriction system is the effectiveness or flow rate changes are dependent on the amount of debris trapped in the system.
Other more sophisticated approaches relying on complete systems can be found in U.S. Pat. Nos. 6,783,683; 6,638,424; 5,707,527; 5,549,817 and 5,322,629 none of which teach a way to achieve such a long draw down time as 48 hours or longer to achieve water quality volume.
A feature of the present invention is it is adaptable to any pre-existing storm catch basin system
Another feature of the present invention is that it permits empirical analysis and verification of the draw down rate.
Another feature is the present invention can be adjusted or modified to increase or decrease the rate of draw down after it has been installed to insure regulatory conditions are precisely met without difficulty.
Changes in future regulatory draw down rates can be easily implemented.